How do I create a custom walker that displays this submenu?

How do I create a custom walker that displays this submenu?

I'm looking to display a menu that mirrors my primary menu as a submenu in
the sidebar. Suppose my site menu is:
Page A
Page B
-Subpage A
-Subpage B
--SubSubpage A
-Subpage C
Page C
If you're on Page B, I'd want this to show:
Page B
-Subpage A
-Subpage B
-Subpage C
If you're on Subpage B, I'd want this to show:
Page B
-Subpage A
-Subpage B
--SubSubpage A
-Subpage C
I know I can do this with a custom walker, but after hours of searching, I
need help.